This application claims priority to Finnish Application No. 19992096, filed Sep. 29, 1999.
The present invention concerns a winter tire for a vehicle, particularly a traction winter tire for heavy trucks, the tread of which is provided with tread pattern blocks and with mainly circumferential grooves separating said pattern blocks from each other and those in transvense direction thereto, the tread pattern block rows between which circumferential grooves are displaced relative to each other so that the adjacent tread pattern blocks in axial direction to the tire are overlapping, and which tread pattern blocks contain sipes opening into the tread and extending into the tire to increase the grip of the tread with the road surface. The purpose of the sipes provided in the tread pattern blocks is to increase the grip of the tires in winter conditions. In the Nordic countries, so-called snow tires provided with open patterns have been used for improving the grip of the tires in winter conditions, the surface-groove ratio whereof being small compared with ordinary traction tires.
A drawback related to said snow tires lies in that the amount of the sipes of the tread cannot be substantially increased without increasing the surface-groove ratio or without reducing the groove depth, both of which decrease the driving properties in snow conditions. If the amount of the sipes is increased and the surface-groove ratio and the groove depth are maintained, the stability of the tire will be impaired and its wear resistance will be degraded
The objective of the present invention is to increase the amount of the sipes without altering the openness of the tread pattern and its groove depth.
The aim has been successfully achieved by means of a winter tire of the invention. The invention is based on the feature that the tread pattern blocks are joined in pairs mainly by an axial bridge part of the tire so that a double tread pattern block is formed comprising mainly an axial bridge zone and the ends (the wing parts) of both tread pattern blocks being joined. In a double tread pattern block of such configuration, at least one sipe of the length of the bridge zone can be formed in the bridge zone, and in addition, at least two sipes of the length of one tread pattern block in the wing parts of the double tread pattern block.
By means of the truck winter tire of the invention, provided with a double tread pattern block, an improved grip can be achieved on an icy and snowy road, that is, in conditions of packed slippery snow, while the wear resistance of the tire is excellent and the shape of the wear is uniform.
The tire of the invention works excellently without anti-skid studs, though studs may also be attached thereon. The patterning of the tire enables an appropriate distribution of studs. An intact cylindrical support area of a tread pattern block is provided. When a stud is provided, the cylindrical support area surrounds the stud. A sipe entering the support area is discontinued on that spot, whereby a lamella used to form an intact sipe is replaced by lamella parts forming a discontinued sipe.
The tread of the tire of the invention may comprise a variable number of tread pattern block rows, adjacent tread pattern blocks being joined in pairs into double tread pattern blocks. A winter traction tire of a conventional-sized heavy truck is preferably provided with six rows of relatively small-sized tread pattern blocks, whereby three overlapping adjacent double tread pattern blocks are formed, or respectively, two double tread pattern blocks and two single tread pattern blocks, i.e. unbound blocks, in the shoulder areas.
If a tire includes an odd number of tread pattern block rows, for instance five or seven, adjacent overlapping double tread pattern blocks and one single tread pattern block are comprised in each axial row, alternately in one shoulder area and alternately in the other shoulder area. As regards the amount of sipes, this does not bear any significance.
Both the single tread pattern blocks in the shoulder area and a tread pattern block of the double tread pattern blocks in the shoulder area are advantageously rectangular (parallelogram) tread pattern blocks, the circumferential edges of which are parallel to the direction of rotation and the transversal edges of which are in axial direction to the tire. The tread pattern blocks of the middle area of the tread form advantageously a small angle with the tire axle so that the transversal edges thereof form an about 0 to 10xc2x0 angle, preferably about 7xc2x0, to the axle, while the circumferential edges are parallel to the direction of rotation.
A double tread pattern block comprises preferably four short sipes, that is sipes, extending across each wing part of the tread pattern block, and one long sipe, extending across the bridge zone and, therefore, being longer by the length of the bridge part than the two short sipes together. The inner sipes of the wing parts are preferably positioned in the immediate proximity of the bridge zone. For reasons of manufacturing a tread pattern block, the side edges are curved inwards, which aids embedding the inner lamellac in the mould.
Since the spaces between the sipes in a double tread pattern block are advantageously mutually equal and since the inner sipes of the wing parts are located in the immediate proximity of the bridge zone, two sipe spaces form the width of the bridge part.
The tire of the invention and the tread thereof are manufactured in a manner obvious to a person skilled in the state of the art. The sipes a re achieved with lamellae of the shape corresponding to the shape of the sipes, which are embedded in the mould. The sipes are straight at the ends of the sipe in top view and Z-shaped or undulated in configuration in the portion of the sipe between the ends of the sipe.
The winter tire of the invention and the lamella used far manufacturing same are described below by an embodiment, reference being made to the accompanying figures, in which